Causes

Tetralogy of Fallot occurs during fetal growth, when the baby's heart is developing. While factors such as poor maternal nutrition, viral illness or genetic disorders may increase the risk of this condition, in most cases the cause of tetralogy of Fallot is unknown.

The four abnormalities that make up the tetralogy of Fallot include:

Pulmonary valve stenosis. This is a narrowing of the pulmonary valve, the flap that separates the right ventricle of the heart from the pulmonary artery, the main blood vessel leading to the lungs. Constriction of the pulmonary valve reduces blood flow to the lungs. The narrowing may also affect the muscle beneath the pulmonary valve.

Ventricular septal defect. This is a hole in the wall that separates the two lower chambers (ventricles) of the heart. The hole allows deoxygenated blood in the right ventricle — blood that has circulated through the body and is en route to the lungs to replenish its oxygen supply — to flow into the left ventricle and mix with oxygenated blood fresh from the lungs. Blood from the left ventricle also flows back to the right ventricle in an inefficient manner. This ability for blood to flow through the ventricular septal defect dilutes the supply of oxygenated blood to the body and eventually can weaken the heart.

Overriding aorta. Normally the aorta, the main artery leading out to the body, branches off the left ventricle. In tetralogy of Fallot, the aorta is shifted slightly to the right and lies directly above the ventricular septal defect. In this position the aorta receives blood from both the right and left ventricles, mixing the oxygen-poor blood from the right ventricle with the oxygen-rich blood from the left ventricle.

Right ventricular hypertrophy. When the heart's pumping action is overworked, it causes the muscular wall of the right ventricle to enlarge and thicken. Over time this may cause the heart to stiffen, become weak and eventually fail.

Rarely, some babies who have tetralogy of Fallot will have a hole between their heart's upper chambers (atrial septal defect), as well. When this occurs, the condition is known as pentalogy of Fallot.

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